In recent years, optical communication lines are spreading to general households as well as being used as backbone lines connecting cities. Making optical communications available in general households requires optical transmitter/receiver modules in every household for the conversion between optical and electrical signals. An optical transmitter/receiver module generally consists of three components: an optical transmitter, optical receiver, and wavelength multiplexer/demultiplexer. Typically, a laser diode (LD) is used as the optical transmitter and a photodiode (PD) is used as the optical receiver. For the wavelength multiplexer/demultiplexer, a flat glass coated with dielectric multilayer films or a cubic prism type has been used so far. However, these devices are relatively large, and a smaller type of wavelength multiplexer/demultiplexer is in demand for use in general households.
Recently, photonic crystals have been drawing attention as new optical devices. A photonic crystal consists of a dielectric body having an artificial cyclic structure. In general, the cyclic structure is created by providing the dielectric body with a cyclical arrangement of areas (called the modified refractive index areas) whose refractive index differs from that of the body. This cyclic structure forms a band structure within the crystal with respect to the energy of light and thereby creates an energy region (called the photonic bandgap or PBG) that disallows the propagation of light. The energy region (or wavelength band) in which the PBG is created depends on the refractive index of the dielectric body and the cycle (cycle distance) of the cyclic structure.
Introduction of an appropriate defect into the photonic crystal creates a specific energy level (called the defect level) within the photonic bandgap, which allows only a ray of light having a wavelength corresponding to the defect level to exist in proximity to the defect. Provision of a point-like defect in a photonic crystal enables the crystal to be used as an optical resonator (i.e. a point-like defect) for the aforementioned wavelength, and provision of a defect extending along a line makes the crystal available as a waveguide (i.e. a linear defect). Furthermore, if the resonator is located in proximity to the waveguide, the photonic crystal functions as a wavelength multiplexer/demultiplexer. This wavelength multiplexer/demultiplexer is capable of functioning as the following two devices: an optical demultiplexer for extracting a ray of light whose wavelength equals the resonance wavelength of the resonator from rays of light having different wavelengths and propagating through the waveguide, and for emitting the extracted light to the outside; and an optical multiplexer for introducing the same ray of light from the outside into the waveguide. Wavelength multiplexers/demultiplexers using a photonic crystal can be smaller in size than the conventional ones using a multilayer-coated flat glass or a cubic prism.
Patent Document 1 discloses such a wavelength multiplexer/demultiplexer, which can multiplex or demultiplex a predetermined wavelength of light by appropriately setting the size and/or shape of the point-like defect. Patent Document 2 discloses a two-dimensional photonic crystal wavelength multiplexer/demultiplexer consisting of a two-dimensional photonic crystal having multiple zones (forbidden band zones) with different cycles, in which a waveguide passes through the multiple zones and a resonator is located in each forbidden band zone. In this construction, each resonator multiplexes or demultiplexes light having a different wavelength due to the different cycle of each zone.
[Patent Document 1] Unexamined Japanese Patent Publication No. 2001-272555 (paragraphs 0023-0027; FIG. 1)
[Patent Document 2] Unexamined Japanese Patent Publication No. 2003-279764 (paragraphs 0029-0034, 0057-0059; FIGS. 17, 18)
The point-like defects formed in the two-dimensional photonic crystal causes the resonation of light within a narrow wavelength band. Although the emission band of laser diodes is also very narrow, it is difficult to produce a product that emits light whose central wavelength coincides exactly with the desired wavelength because the central wavelength varies due to productive factors or other reasons. Therefore, to apply a two-dimensional photonic crystal wavelength multiplexer/demultiplexer to an optical transmitter/receiver module, it is necessary to perform a tuning (or selection) of laser diodes so that the wavelength used by the laser diode falls within the narrow wavelength band of the point-like defect resonator. Such a tuning process enables the two-dimensional photonic crystal wavelength multiplexer/demultiplexer having a point-like defect to be used to construct a high-precision optical transmitter/receiver module for long distance support systems.
However, as for household optical transmitter/receiver modules, the tuning process is difficult to perform in view of its costs. Therefore, it is desirable to invent a wavelength multiplexer/demultiplexer capable of multiplexing or demultiplexing light within a wavelength band having a certain width so that it can accommodate various wavelengths of laser diodes to be used.